In radio receivers, particularly in mobile and high gain FM communication receivers, it is well known to utilize squelch circuits to automatically cut off the audio output in the absence of a received carrier in order to prevent annoying receiver noise from being audible during intervals between signal reception. Upon the reception of a reasonable strength carrier signal following an interval of silence, the squelch circuit reactivates the audio output, allowing the signal to be heard as long as it is present. However, prior art squelch circuits for controlling the audio output of radio receivers, particularly FM scanning receivers, typically suffer from the problem of having long turn-on and turn-off delay times. This can produce an initial loss of audio output upon reception of weaker transmissions and allows an annoying burst of noise or "squelch-tail" at the end of a transmission. Although squelch circuits having a very short time constant could be used, they are problematic because the squelching operation of the receiver then becomes overly sensitive to any temporary or rapid variations in the strength of the received signal, commonly referred to as "flutters" or "fades", which cause the squelch to produce annoying intermittent interruptions in the audio output.
In a scanning receiver or in "priority search" type applications, a conventional noise squelch system would discriminate against low signal level transmissions, since the squelch operation is too sluggish, whereas a very fast acting squelch would require a large amount of hysteresis for satisfactory operation. It is thus desirable to provide a fast acting squelch circuit which does not interrupt the audio during temporary fluctuations in signal strength due to flutters or fades and which also prevents the occurrence of a long squelch tail at the end of strong transmissions. The present invention addresses these problems and is particularly applicable toward achieving fast squelch operation in FM receivers operating in an intermittent mode (e.g., a power saving mode) or in scanning receivers.
In FM receivers, the receiver noise above the audio band is commonly used to indicate the presence of a carrier signal. A suitable threshold value for the noise level is established for a receiver (depending on its particular signal to noise ratio characteristics) and is used to determine when to enable or disable the receiver's audio output. A block diagram of a conventional prior art noise squelch system of this sort is illustrated in FIG. 1. Typically, the above audio band receiver noise is filtered to remove the lower frequency speech components, then amplified and rectified. This detected noise voltage is subsequently integrated and the integration output is then compared with a DC squelch threshold reference voltage by a comparator. The decision output of the comparator is then used to control a muting switch in the audio output path.
Referring to the prior art example illustrated in FIG. 1, the signal from a radio receiver FM demodulator 10 is typically filtered by a high-pass filter or a band-pass filter 12 to extract the above-audio band noise. This noise signal is amplified by the noise amplifier 14 and rectified by noise rectifier 16 to produce a DC voltage output. This detected noise voltage can be used as a measure of the strength of a received FM carrier signal. If the detected voltage is directly related to the above audio band noise level, then it will be inversely related to the received FM signal strength. Accordingly, noise rectifier 16 is often designed to produce a detected voltage that is inversely related to the noise level so that it will be directly related to FM signal strength. A prior art squelch circuit of this general sort is disclosed by U.S. Pat. No. 4,359,780 to Day (1982).
In the FIG. 1 depiction of a typical prior art circuit, the DC voltage output from noise rectifier 16 is applied to input 17 of squelch control circuit 20. Within squelch control circuit 20, the detected noise voltage is filtered to eliminate low-frequency DC fluctuations and integrated by low-pass filter/integrator 18. The integrated signal is compared to a predetermined squelch threshold voltage by comparator 19, and the decision output 11 is used to indicate RF carrier activity.
Additional exemplary prior art squelch circuit configurations are disclosed in the following U.S. Patents:
U.S. Pat. No. 3,769,592 to Espe (1973)
U.S. Pat. No. 3,979,679 to Bush et al (1976)
U.S. Pat. No. 4,085,370 to Vanderpole et al (1978)
U.S. Pat. No. 4,132,953 to Martin, III (1979)
U.S. Pat. No. 4,176,286 to Shuffield, Jr. (1979)
U.S. Pat. No. 4,479,250 to Flood (1984)
U.S. Pat. No. 4,731,868 to Dreier (1988)
U.S. Pat. No. 4,947,456 to Atkinson et al (1990)
U.S. Pat. No. 4,972,510 to Guizerix et al (1990)
Usually, the comparator in a squelch control circuit is designed with some positive feedback to minimize any "dithering" of the squelch output about a pre-set reference threshold noise level caused by fluctuations in the rectified detected noise output. This is accomplished by providing hysteresis in the setting of the squelch comparator threshold level so that the receiver will unsquelch at a higher level input RF signal strength than it takes to squelch it. However, in order for a squelch circuit to operate as rapidly as possible, it is essential that the filtered and integrated output of the noise rectifier reach its final value quickly. This implies using a relatively wide bandwidth low-pass filter with the filter/integrator 18 in the squelch control circuit. Unfortunately, the use of a relatively wide bandwidth filter also inherently allows relatively large fluctuations in the integrator DC output. This necessitates designing the squelch circuit comparator with a large amount of hysteresis to keep the squelch from "bobbling" around a single set reference threshold noise level. However, a large amount of hysteresis is undesirable since it reduces the effective sensitivity of the receiver and allows annoying squelch-tail noise bursts at the end of transmissions and, in a scanning type of receiver context, may cause one to entirely bypass lower level RF carriers.